Why do people forget names but remember songs from twenty years ago? Why does a crowd sometimes stand by while someone is hurt? Why do two children raised in the same house turn out so differently? Questions like these are as old as human beings, and for most of history they belonged to philosophers, priests and storytellers. Psychology is the attempt to answer them with evidence. Its usual definition is simple: the scientific study of behavior and mental processes. Behavior is what we can observe, such as a word spoken or a button pressed. Mental processes are the thoughts, feelings, memories and perceptions behind it, which we cannot see directly but can study through careful measurement.

Old questions, new methods

Greek thinkers argued about whether knowledge is inborn or learned, and Aristotle wrote a whole treatise on the soul and the senses. Seventeenth-century philosophers revived the debate: Descartes treated the mind as separate from the body, while John Locke described the newborn mind as a blank slate written on by experience. These arguments still echo in what psychologists call the nature and nurture question. What changed in the nineteenth century was not the questions but the tools. Physiologists began measuring reaction times and the limits of the senses, and it became possible to treat the mind as something that could be tested rather than only reasoned about.

Leipzig, 1879

Most histories date modern psychology to 1879, when Wilhelm Wundt opened a laboratory for experimental psychology at the University of Leipzig in Germany. Wundt and his students measured how long it took people to react to a sound or a light, and trained volunteers to report their own sensations in careful detail, a method called introspection. Introspection turned out to be unreliable, since different labs trained observers who reported different things, but the idea of a psychology laboratory spread quickly. In the United States, William James published The Principles of Psychology in 1890, a book still quoted for its vivid writing about habit, attention and the stream of consciousness. One of his students, Mary Whiton Calkins, completed all the requirements for a Harvard doctorate, was refused the degree because she was a woman, and went on to become the first woman president of the American Psychological Association in 1905.

Schools of thought

Early psychology split into rival schools, each with a different idea of what the field should study:

• Structuralism, associated with Wundt's student Edward Titchener, tried to break conscious experience into basic elements.
• Functionalism, inspired by James and by Darwin, asked what mental processes are for and how they help us adapt.
• Psychoanalysis, founded by the Viennese doctor Sigmund Freud, stressed unconscious wishes and childhood conflict. Many of its specific claims have not held up, but the idea that much of mental life happens outside awareness has.
• Behaviorism, launched by John B. Watson in 1913, rejected talk of the mind altogether and studied only observable behavior and how it is learned.
• Humanistic psychology, led by Abraham Maslow and Carl Rogers in the 1950s, pushed back against both Freud and the behaviorists by emphasizing growth, choice and the whole person.

From the 1950s onward the cognitive revolution brought the mind back. Inspired partly by early computers, researchers began studying memory, language and problem solving as forms of information processing. In 1959 the linguist Noam Chomsky published a famous review arguing that behaviorism could not explain how children learn language. Today most psychologists borrow from several of these traditions rather than belonging to one.

Levels of analysis

A useful modern habit is to look at any behavior from several levels at once. The biopsychosocial approach asks about biological factors such as genes, hormones and brain activity; psychological factors such as thoughts, emotions and learned habits; and social factors such as family, culture and economic conditions. Depression, for example, can involve all three: an inherited vulnerability, a habit of interpreting setbacks as permanent, and the loss of a job or a friend. No single level tells the whole story.

What psychologists actually do

Psychology is a broad field. Clinical and counseling psychologists assess and treat mental health problems. Developmental psychologists study change across the lifespan. Social psychologists study how people influence one another. Others work in schools, workplaces, courts, sports teams and hospitals, or study the brain directly. Psychologists use the same tools of evidence as other sciences, from controlled experiments to large surveys, and like other sciences the field has had to clean up its methods: a large 2015 effort to repeat 100 published studies found that fewer than half produced the same result. You can think of this course as a tour of what has held up.

Worked example: one question, many angles

Suppose a teenager suddenly stops doing homework. Each tradition would look in a different place. A biological psychologist might ask about sleep, since adolescent body clocks shift later and tired brains struggle to focus. A behaviorist would look at what follows homework: is it ever rewarded, or does it only bring criticism? A cognitive psychologist might ask what the student believes, such as "I'm just bad at math, so why try?" A social psychologist would ask about friends, family stress and what peers think of schoolwork. A humanistic psychologist would ask what the student actually wants for their own life. The best answer probably combines several of these. That is the spirit of modern psychology: curiosity about the whole person, disciplined by evidence.